(1) Field of the Invention
The present invention relates to a ceramic heater excellent in the thermal shock resistance and the strength at high temperatures, which can be widely used for ordinary houses, electronic parts, industrial equipments and automobiles.
(2) Description of the Prior Art
As the heater comprising a ceramic material as the substrate, there has been mainly used a heater comprising a resistor composed mainly of a metal such as tungsten (W) or molybdenum (Mo), which is in an alumina (Al.sub.2 O.sub.3) sintered body.
This ceramic heater is advantageous in that it is excellent in the electrically insulating property chemical resistance and abrasion resistance. However, alumina is poor in the thermal shock resistance and the strength at high temperatures. Namely, the thermal shock-resistant temperature difference is about 200.degree. C. when thrown in water and quenched, and the strength at high temperatures of up to 800.degree. C. is about 30 kg/mm.sup.2 as determined by the 4-point bending flexural strength method.
Accordingly, use of a silicon nitride sintered body, which is excellent over other ceramics in the thermal shock resistance and the strength at high temperatures, as the substrate of a heater has attracted attention in the art. This silicon nitride sintered body is superior to alumina in that the thermal shock-resistant temperature difference is about 600.degree. C. and the strength at high temperatures of up to 800.degree. C. (4-point bending flexural strength) is 60 kg/mm.sup.2.
A ceramic heater comprising this silicon nitride sintered body as the substrate, in which a heat-generating resistor metal such as tungsten (W) or molybdenum (Mo) is embedded as in case of the alumina, has already been proposed, and a heater formed by printing a heat-generating resistor paste composed of such a metal as tungsten (W) or molybdenum (Mo) on a silicon nitride green sheet, laminating the green sheet on the printed sheets and sintering the laminate integrally is proposed in Japanese Patent Application Laid-Open No. 55-126989.
However, when a metal such as tungsten (W) or molybdenum (Mo) is used as the heat-generating resistor, at sintering at high temperatures or during long-time application where elevation and dropping of the temperature are repeated, the metal such as tungstem (W) or molybdenum (Mo) reacts with silicon nitride (Si.sub.3 N.sub.4) in the interface between the periphery of the heat-generating resistor and silicon nitride and a layer of WSi.sub.2 or MoSi.sub.2 is readily formed. Furthermore, a layer of WO.sub.3 or MoO.sub.3 is readily formed by reaction with oxygen. Since the so-formed reaction layers are physically brittle, the dispersion of the resistance value is large, and especially in case of a high-resistance heater, cracks are readily formed in the interface where the reaction layers are formed and breaking of the heat-generating resistor is caused. Because of these defects, the conventional heaters, especially the heaters formed by using a heat-generating resistor paste, are hardly put into practical use. A heat-generating resistor composed of a metal such as tungsten (W) or molybdenum (Mo) has a relatively high resistance temperature coefficient (TCR) of about 4.times.10.sup.-3 to about 5.times.10.sup.-3 (0.degree. to 800.degree. C.). Accordingly, even in the practically used heater having a heat-generating resistor metal such as tungsten (W) or molybdenum (Mo) embedded in the substrate, the inrush current is increased at the time of application of the voltage, and an electricity control apparatus in which the current capacity is large is necessary for the heater. In a heat-generating resistor composed of a metal such as tungsten (W) or molybdenum (Mo), the change of the resistance according to the temperature is not linear and the temperature is not constantly elevated with rise of the voltage. Accordingly, it is difficult to perform the temperature control by detecting the resistance value.